CONNECT(3SOCKET) Sockets Library Functions CONNECT(3SOCKET)


connect - initiate a connection on a socket


cc [ flag ... ] file ... -lsocket -lnsl [ library ... ]
#include <sys/types.h>
#include <sys/socket.h>

int connect(int s, const struct sockaddr *name, int namelen);


The parameter s is a socket. If it is of type SOCK_DGRAM, connect()
specifies the peer with which the socket is to be associated. This
address is the address to which datagrams are to be sent if a receiver is
not explicitly designated. This address is the only address from which
datagrams are to be received. If the socket s is of type SOCK_STREAM,
connect() attempts to make a connection to another socket. The other
socket is specified by name. name is an address in the communication
space of the socket. Each communication space interprets the name
parameter in its own way. If s is not bound, then s will be bound to an
address selected by the underlying transport provider. Generally, stream
sockets can successfully connect() only once. Datagram sockets can use
connect() multiple times to change their association. Datagram sockets
can dissolve the association by connecting to a null address.

Non-blocking Sockets
When a socket is created, it is by default a blocking socket. A socket
may be configured to be non-blocking either at socket creation time or
through the use of fcntl(2). When a socket is set to be non-blocking, a
call to connect initiates an asynchronous connection. If the connection
cannot be completed without blocking, such as when making a TCP
connection to a remote server, then the connection attempt is made in
the background and connect returns -1 and errno is set to EINPROGRESS.

Applications can obtain the state of this connection attempt by polling
the socket's file descriptor for POLLOUT. The event ports facility is the
preferred means of polling on the file descriptor, see port_create(3C)
and port_get(3C) for more information on event ports; however,
applications may also use traditional portable routines like poll(2) and

When an asynchronous connection has completed, the application must call
getsockopt(3SOCKET) using the macro SOL_SOCKET as the level argument and
the macro SO_ERROR as the value of the option argument. If the value of
the SO_ERROR socket option is zero, then the connect was successfully
established. Otherwise, the connection could not be established and the
value is the corresponding error code that would be commonly found in

Even when a socket is in non-blocking mode, a call to connect may fail
synchronously. If any error other EINPROGRESS or EINTR occurs, then there
is no need for the application to poll for asynchronous completion.
Similarly, if a call to connect returns successfully, then the socket
connection will be established and there is no need to poll for


Example 1: Performing an asynchronous connection

The following sample C program shows how to create and connect to a
remote host using TCP. The program should be compiled and linked against
libnsl and libsocket. For example, if the contents of this example where
in a file called example.c, one would run cc example.c -lnsl -lsocket.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <inttypes.h>
#include <stdio.h>
#include <strings.h>
#include <stdlib.h>
#include <errno.h>
#include <port.h>
#include <unistd.h>
#include <assert.h>

main(int argc, char *argv[])
char *eptr;
long port;
int sock, ret, eport;
struct sockaddr_in6 sin6;

if (argc != 3) {
fprintf(stderr, "connect: <IP> <port>\n");
return (1);

bzero(&sin6, sizeof (struct sockaddr_in6));
sin6.sin6_family = AF_INET6;

* Try to parse as an IPv6 address and then try v4.
ret = inet_pton(AF_INET6, argv[1], &sin6.sin6_addr);
if (ret == -1) {
return (1);
} else if (ret == 0) {
struct in_addr v4;
ret = inet_pton(AF_INET, argv[1], &v4);
if (ret == -1) {
return (1);
} else if (ret == 0) {
fprintf(stderr, "connect: %s is not a valid "
"IPv4 or IPv6 address\n", argv[1]);
return (1);
/* N.B. Not a portable macro */
IN6_INADDR_TO_V4MAPPED(&v4, &sin6.sin6_addr);

errno = 0;
port = strtol(argv[2], &eptr, 10);
if (errno != 0 || *eptr != ' ') {
fprintf(stderr, "failed to parse port %s\n", argv[2]);
return (1);
if (port <= 0 || port > UINT16_MAX) {
fprintf(stderr, "invalid port: %ld\n", port);
return (1);
sin6.sin6_port = htons(port);

sock = socket(AF_INET6, SOCK_STREAM | SOCK_NONBLOCK, 0);
if (sock < 0) {
return (1);

eport = port_create();
if (eport < 0) {
(void) close(sock);
return (1);

ret = connect(sock, (struct sockaddr *)&sin6,
sizeof (struct sockaddr_in6));
if (ret != 0 && errno != EINPROGRESS && errno != EINTR) {
(void) close(sock);
(void) close(eport);
return (1);

if (ret != 0) {
port_event_t pe;
int err;
socklen_t sz = sizeof (err);
if (port_associate(eport, PORT_SOURCE_FD, sock, POLLOUT,
NULL) != 0) {
(void) close(sock);
(void) close(eport);
return (1);
if (port_get(eport, &pe, NULL) != 0) {
(void) close(sock);
(void) close(eport);
return (1);
assert(pe.portev_source == PORT_SOURCE_FD);
assert(pe.portev_object == (uintptr_t)sock);
if (getsockopt(sock, SOL_SOCKET, SO_ERROR, &err, &sz) != 0) {
(void) close(sock);
(void) close(eport);
return (1);
if (err != 0) {
/* Asynch connect failed */
fprintf(stderr, "asnchronous connect: %s\n",
(void) close(sock);
(void) close(eport);
return (1);

/* Read and write to the socket and then clean up */

return (0);


If the connection or binding succeeds, 0 is returned. Otherwise, -1 is
returned and sets errno to indicate the error.


The call fails if:

Search permission is denied for a component of the path
prefix of the pathname in name.

The address is already in use.

The specified address is not available on the remote

Addresses in the specified address family cannot be used
with this socket.

The socket is non-blocking, and a previous connection
attempt has not yet been completed.

s is not a valid descriptor.

The attempt to connect was forcefully rejected. The
calling program should close(2) the socket descriptor,
and issue another socket(3SOCKET) call to obtain a new
descriptor before attempting another connect() call.

The socket is non-blocking, and the connection cannot be
completed immediately. See the section on Non-blocking
Sockets for more information.

The connection attempt was interrupted before any data
arrived by the delivery of a signal. The connection,
however, will be established asynchronously.

namelen is not the size of a valid address for the
specified address family.

An I/O error occurred while reading from or writing to
the file system.

The socket is already connected.

Too many symbolic links were encountered in translating
the pathname in name.

The network is not reachable from this host.

The remote host is not reachable from this host.

A component of the path prefix of the pathname in name
does not exist.

The socket referred to by the pathname in name does not

There were insufficient STREAMS resources available to
complete the operation.

The server exited before the connection was complete.

Connection establishment timed out without establishing
a connection.

The socket is marked as non-blocking, and the requested
operation would block.

The following errors are specific to connecting names in the UNIX domain.
These errors might not apply in future versions of the UNIX IPC domain.

A component of the path prefix of the pathname in name is
not a directory.

s is not a socket.

name is not a socket.

The file that is referred to by name is a socket of a type
other than type s. For example, s is a SOCK_DGRAM socket,
while name refers to a SOCK_STREAM socket.


See attributes(5) for descriptions of the following attributes:

|MT-Level | Safe |


close(2), accept(3SOCKET), getsockname(3SOCKET), select(3C),
socket(3SOCKET), sockaddr(3SOCKET), socket.h(3HEAD), attributes(5)

November 25, 2014 CONNECT(3SOCKET)